1. Field of the Invention
The present invention relates to a method of detecting positions which is suitable for positioning of a photosensitive substrate through an exposure apparatus employed when manufacturing, e.g., a large-sized liquid crystal display element substrate, etc.
2. Related Background Art
A projection exposure apparatus has hitherto been employed for manufacturing a liquid crystal display element or a semiconductor element by use of photolithography. The projection exposure apparatus projects a reticle pattern on a substrate coated with a photosensitive substance on a stage through a projection optical system. Generally, the liquid crystal display element is manufactured by forming multi-layered circuit patterns in superposition on the substrate. On that occasion, it is required that the circuit pattern of the second layer be superposed on, e.g., the circuit pattern of the first layer at a high accuracy. This in turn requires a highly accurate alignment of the 1st-layer circuit pattern already formed on the photosensitive substrate with the reticle formed with a transfer circuit pattern for the second layer when effecting a shot of, e.g., the 2nd-layer circuit pattern on the photosensitive substrate.
One of such alignment methods of the photosensitive substrate is an alignment method based on an image processing system involving pattern matching.
FIG. 3 schematically illustrates a construction of the projection exposure apparatus for manufacturing a large-sized liquid crystal display element substrate in which the alignment is effected according to the conventional image processing system. Referring to FIG. 3, beams of exposure light emitted from a light source 1 such as a mercury-arc lamp, etc. are converged through an elliptical mirror 2. The beams thereafter fall on a condenser lens system 3 via an unillustrated optical integrator or the like. A reticle R is illuminated with the beams of exposure light with a substantially uniform illuminance which have properly been converged by the condenser lens system 3. With this exposure light, a pattern of the reticle R is subjected to a projection-shot on each shot field on a glass plate 4 coated with a photosensitive substance and serving as a photosensitive substrate through a projection optical system PL. The multi-layered circuit patterns are formed in superposition on the glass plate 4, thereby manufacturing the large-sized liquid crystal display element substrate.
This glass plate 4 is held on a Z-stage 5. The Z-stage 5 is mounted on an XY-stage 6. The XY-stage 6 performs positioning of the glass plate 4 within a plane (XY plane) perpendicular to the optical axis of the projection optical system PL. The Z-stage 5 effects position of the glass plate 4 in the optical-axis (Z-axis) direction of the projection optical system PL. Note that a .theta.-table for turning the glass plate 4 is, though not shown, interposed between the Z-stage 5 and the glass plate 4. Further, a fiducial mark aggregate 7 formed with a variety of alignment marks is fixed in the vicinity of the glass plate 4 on the Z-stage 5. Further, X- and Y-directional moving mirrors 8 are fixed thereto. The numeral 9 designates a biaxial laser interferometer. The numeral 10 represents a driving unit. Laser beams from the laser interferometer 9 are reflected by the moving mirror 8. Coordinates of the XY-stage 5 are always measured by the laser interferometer 9. The driving unit 10 drives the XY-stage 6 in accordance with a coordinate value measured by the laser interferometer 9.
The numeral 11 stands for an alignment scope. An alignment mark RM existing in the vicinity of a pattern field of the reticle R is irradiated with alignment light from the alignment scope 11 when aligning the reticle R. The light reflected from this alignment mark RM is further reflected by a mirror 12 and returns to the alignment scope 11. Therefore, the reticle R is aligned by adjusting a position of the reticle R on the basis of an image position of the alignment mark R which is re-formed inwardly of the alignment scope 11. Further, the alignment mark RM of the reticle R and the alignment mark within the fiducial mark aggregate 7 on the Z-stage 5 are simultaneously viewed through the alignment scope 11. The reticle R may be aligned based on a positional relationship between two images thereof. Besides, the positional relationship therebetween can be also obtained by simultaneously viewing the alignment mark of the reticle R and the alignment mark on the glass plate 4 through the alignment scope 11.
The numeral 13 denotes a light sending system of an autofocus detection system. The numeral 14 represents a light receiving system of the autofocus detection system. An image of a detection pattern such as a slit pattern is projected on the glass plate 4 from the light sending system 13 obliquely to an optical axis AX of the projection optical system PL. The image of the detection pattern is re-formed within the light receiving system 14 by the reflected light from the detection pattern image. A height of an exposure surface of the glass plate 4 is obtained from a positional shift quantity of the re-formed image of the detection pattern. The height of the exposure surface of the glass plate 4 is set to a best focus position with respect to the projection optical system PL by means of the Z-stage 5.
Further, an image processing alignment optical system is disposed sideways of the projection optical system PL. In this alignment optical system, the numeral 15 designates an objective lens. The reflected light from a viewing area illuminated with the light coming from an unillustrated illumination system of the glass plate 4 is incident on a conjugate index plate 17 exhibiting a light transmitting property via the objective lens 15 and a mirror 16. Index marks are depicted on the conjugate index plate 17. The exposure surface of the glass plate 4 is conjugate to a depiction surface of the index marks of the conjugate index plate 17. A pattern image of the viewing area of the glass plate 4 is formed on the depiction surface. Further, the beams of light penetrating the conjugate index plate 17 are converged at an imaging plane of the CCD camera 19 which uses a charge coupled imaging device (CDD) through a relay lens 18. Formed on the imaging plane thereof are the image of the viewing area pattern of the glass plate 4 and the index mark image of the conjugate index plate 17. A video signal (imaging signal) outputted from the CCD camera 19 is supplied to a pattern matching unit 20.
FIG. 4A shows one example of the patterns on the glass plate 4 in FIG. 3. As illustrated in FIG. 4A, two liquid crystal pixel segments 21A, 21B are formed on the glass plate 4 by processes conducted so far. A single or a plurality of circuit patterns are formed on the respective liquid crystal pixel segments 21A, 21B. Further, a plurality of cross alignment marks 22-1, 22-2, . . . , 22-9 intrinsic to the apparatus are formed along the peripheries of the liquid crystal pixel segments on the glass plate 4.
Then, when executing a shot of a circuit pattern of the next layer, i.e., the pattern of the reticle R on the glass plate 4, the alignment marks 22-1 to 22-9 on the glass plate 4 are detected by pattern matching based on a normalized correlation method or the like. Specifically, when detecting the alignment mark 22-6 in FIG. 4A, a viewing area embracing the alignment mark 22-6 of the glass plate 4 is set downwardly of the objective lens 15 by driving the XY-stage 6 in FIG. 3. FIG. 4B illustrates a processed image 23-6 to be imaged by the CCD camera 19 on such an occasion. This processed image 23-6 includes an image 22-6G of the alignment mark 22-6.
Further, FIG. 4C shows a reference image 25. This reference image 25 includes a specified mark 24 having the same shape as those of the alignment marks 22-1 to 22-9 in FIG. 4A. The reference image 25 and the specified mark 24 are obtained by imaging video signals as templates stored in an internal memory of the pattern matching unit 20. In this pattern matching unit 20, a video signal corresponding to a sampling image having the same size as that of the reference image 25 in the processed image 23-6 undergoes pattern matching with a video signal (template) of the reference image 25. Detected from the processed image 23-6 is an image 22-6G of the same alignment mark as the specified pattern 24 of the reference image 25. Obtained is a positional shift quantity of the image 22-6G with respect to a design position in which the image 22-6G within the processed image 23-6 should exist in the coordinates of the XY-stage 6 in FIG. 3 in this case. Coordinates of the alignment mark 22-6 within the XY plane are acquired.
Similarly, other alignment marks 22-1 to 22-5 and 22-7 to 22-9 in FIG. 4A are detected by the pattern matching, and coordinates thereof are also obtained. Subsequently, design coordinates of the alignment marks 22-1 to 22-9 on the glass plate 4 are compared with the actually measured coordinates. A positional shift quantity of the glass plate 4 is thereby calculated. A shot of the pattern of the reticle R is performed while positioning the respective shot fields on the glass plate 4. This is effected based on a coordinate system obtained by correcting the positional shift quantity with respect to the coordinate system on the glass plate 4.
According to the conventional projection exposure apparatus, however, the alignment marks 22-1 to 22-9 intrinsic to the apparatus employed for the alignment can not be disposed inwardly of the liquid crystal pixel segments 21A, 21B. These alignment marks can only be arranged along the peripheries of the liquid crystal pixel segments 21A, 21B. Hence, positional shifts of the circuit patterns themselves within the liquid crystal pixel segments 21A, 21B can not be directly measured. This results in an offset in the coordinate system on the glass plate. This in turn leads to such an inconvenience that a superposing accuracy (matching accuracy) between, e.g., the 1st-and 2nd-layer circuit patterns decreases. In particular, the offset in the coordinate system increases with larger image fields of the liquid crystal pixel segments 21A, 21B. The drop in the superposing accuracy thereof is not ignorable.
Additionally, a conceivable measure for correcting the offset in the coordinate system is to increase the number of the alignment marks disposed along the peripheries of the liquid crystal pixel segments 21A, 21B on the glass plate 4. If the image fields of the liquid crystal pixel segments 21A, 21B are increasingly enlarged, however, it is difficult to precisely estimate internal offsets of the liquid crystal pixel segments 21A, 21B. Further, the alignment marks shown in FIG. 4A are not all disposed along the periphery of the glass plate 4. Consequently, there is a possibility of easily receiving an influence of the manufacturing process. The alignment marks also undergo a relatively large damage when forming the multi-layered circuit patterns. The alignment marks are therefore hard to precisely detect, and there is a possibility in which an accuracy of measuring the offset in the coordinate system worsens.